1. Field of the Invention
The present invention relates, in general, to linear tape systems, and, more particularly, to systems and methods for slimming tape servo pulses using non-linear gain.
2. Relevant Background
Because of its relatively low cost, linear tape is commonly used as a medium for storing large amounts of digital data for archival purposes. For example, disk-based memory is often archived on linear data storage tape. Data is formatted on linear tapes in a plurality of tracks that extend longitudinally along the tape. A tape head is moveable laterally across the tape to read or write different tracks. In many cases, multiple tracks can be written or read at the same time by using a tape head with multiple read/write elements. When reading or writing a linear data storage tape, accurate lateral positioning of the tape head is very important. To achieve such accuracy, servo stripes are prewritten on the tape. The servo stripes are detected by the tape head during reading and writing to determine the exact lateral position of the tape head relative to the linear tape.
FIG. 1 illustrates, conceptually, the use of servo stripes. FIG. 1 shows a segment of a linear tape 100 that extends in a longitudinal direction x, and that has a lateral dimension y. The tape includes a plurality of servo stripes 120. In the simplified example of FIG. 1, there are three servo stripes. The servo stripes are written to the tape during a preparatory “formatting” process, prior to actual use of the tape for data storage. The servo stripes are spaced laterally from each other by a specified distance. Data tracks 140 are located between the servo stripes. The lateral positions of the data tracks are specified relative to the servo stripes. When reading or writing on a tape 100, a tape head senses the servo stripes with servo read elements and positions itself precisely relative to the servo stripes. Within the tape head, data read/write elements are spaced relative to the servo read elements so that the data read/write elements will be positioned over data tracks 140 when the servo read elements are positioned accurately over the corresponding servo stripes 120.
There are different ways to derive lateral position information from a servo stripe. One common way is to divide a servo stripe into two half stripes, which are recorded with different information (such as two distinct frequencies or bursts occurring at distinct times). A single servo head straddles the boundary between the half stripes, and position information is obtained by comparing the amplitude or phase responses of the signals generated from the respective half stripes.
FIG. 2 shows an example of a servo pattern, as is known in the prior art, using a continuously-variable, timing-based servo pattern, along with a signal generated by a servo read element positioned over the servo pattern. The pattern consists of alternating magnetic transitions at two different azimuthal slopes. Relative timing of pulses generated by the read element depends on the lateral position of the head. More specifically, the servo stripe illustrated in FIG. 2 has a series of magnetic transitions 200, 220 referred to as “stripes” 200, 220 that are recorded on the tape with alternate azimuthal slopes. Every other stripe 200 shown in FIG. 2 has a positive slope, while the intervening stripes 220 have negative slopes.
FIG. 2 shows the path and width of the servo head, indicated by reference numeral 240. The servo head reads a lateral width that is significantly less than the full lateral width of the stripes themselves. The signal generated by the servo head is represented by trace 260, illustrated directly below the illustrated magnetic transition stripes. As the servo head passes over the leading stripe edge, a positive pulse is developed and as the servo head passes over the trailing stripe edge, a negative pulse is created. Lateral position information can be derived by comparing the distances between pulses and groups of pulses. For example, a first distance A can be defined as the distance from a positive sloped stripe to the next negative sloped stripe, while a second distance B can be defined as the distance from a negative sloped stripe to the next positive sloped stripe. When the servo head is centered over the servo stripe, A will be equal to B: consecutive pulses will occur at equal intervals. In actual implementation, alternating “bursts” of stripes are used, with a burst being defined as one or more individual magnetic transition stripes.
FIG. 3 shows two sections of stripe bursts 310 used to position a servo read head 320. When the servo head 320 passes over the bursts of stripes 310, a series of pulses 330 are generated by the leading and trailing stripe edges. Ideally each pulse would possess a trapezoidal shape 340 with a rapid rise, a short horizontal peak 350 and a rapid linear descent followed by a short horizontal trough 360. In reality, each pulse resembles a waveform that is less than perfect with a shoulder 370 in some fashion as it crosses the baseline. There are many reasons for the imperfection including the inability to precisely and consistently create the stripes, the differing width in the bars and imperfections in the tape itself. Jitter and noise also blur the transition between each positive and negative pulse further reducing the ability to position the servo head accurately.
The accuracy of the transition time, that is the time from the leading edge of the stripe to the trailing edge, is determined by the narrowness of the pulses. The pulses 330 as shown in FIG. 3 are developed from the servo read head 320 and converted to digital pulses that switch on the pulse peaks. Traditionally, a filtering technique called “read equalization” has been used to reduce the impact of unwanted noise and jitter. Such a filter is frequency dependent such that higher and lower frequencies are attenuated differently. A schematic of a typical implementation of a read equalization circuit as used in a servo head reader is shown in FIG. 4. This circuit takes the raw servo signal 330 and amplifies it 440, AC couples 450 with an automatic gain control loop 460 and then applies a low-pass filter 410.
The filter 410 generates components of the original signal, the normal signal 420 and the differentiated signal 430 to accomplish pulse-slimming to narrow the pulses and make peak detection more precise.
Read equalization adds the derivative of the signal to the signal itself to accomplish pulse-slimming in the last stage of the filtering process. Mathematically this necessitates multiplying the signal by a linear constant and a complex frequency. The use of the wave form and a differentiated signal as is accomplished in read equalization increases the inaccuracy of true peak detection due to the presence of the shoulders in the original pulse.
To increase the accuracy of the positioning of a head reader in a tape system, it is desirable to narrow the pulses read by the servo head reader while simultaneously reducing baseline noise, jitter and distortion to increase the precision of peak detection. The present invention offers these and other advantages as is shown with reference to the following diagrams and described in the detailed description.